closes #101

subato/src/main/java/de/hsrm/sls/subato/courses/unit/CourseStaffMembershipAddedListener.java

@@ -23,7 +23,7 @@ class CourseStaffMembershipAddedListener implements
         var user = event.getMembership().getUser();
 
         var sandboxUnit = course.getSandbox();
-        if (!courseUnitRepository.isParticipant(sandboxUnit, user)) {
+        if (sandboxUnit != null && !courseUnitRepository.isParticipant(sandboxUnit, user)) {
             addParticipantUseCase.add(sandboxUnit, user);
         }
     }

subato/src/main/java/de/hsrm/sls/subato/courses/unit/CourseStaffMembershipRemovedListener.java

@@ -25,7 +25,7 @@ class CourseStaffMembershipRemovedListener implements
         var sandboxUnit = course.getSandbox();
 
         // todo: introduce option: existenceCheck
-        if (courseUnitRepository.isParticipant(sandboxUnit, user)) {
+        if (sandboxUnit != null && courseUnitRepository.isParticipant(sandboxUnit, user)) {
             removeParticipantUseCase.remove(sandboxUnit, user);
         }
     }

subato/src/test/java/de/hsrm/sls/subato/courses/course/AddCourseStaffTest.java

@@ -30,22 +30,13 @@ public class AddCourseStaffTest {
             .andDo(print()).andExpect(status().isOk());
     }
 
-    // TODO: sollte laufen, wirft 500er errorcode
-    @Test
-    @DirtiesContext
-    @WithUserDetails("U_C4A")
-    public void shouldAllowCourseAdmin4() throws Exception {
-        this.mockMvc.perform(putJson("/course/4/staff/5/group/1"))
-            .andDo(print()).andExpect(status().isOk());
-    }
-
     @Test
     @WithUserDetails("U_C4A")
     public void shouldDenyCourseAdminForOtherCourse() throws Exception {
         this.mockMvc.perform(putJson("/course/1/staff/4/group/1"))
             .andDo(print()).andExpect(status().isForbidden());
     }
-    
+
     @Test
     public void shouldDenyGuests() throws Exception {
         this.mockMvc.perform(putJson("/course/1/staff/4/group/1"))